ES2 (with external battery) scooter rides slower
 

I bought a new ES2 about 3 months ago and then an external battery about 2 months ago to up it to the ES4 speed, and everything was going great (getting about 17-18 mph on flat streets) but in the last week it's slowed down to about 14/15 mph.

I take the same route to/from work every day (9 miles in total), make sure to charge it to 100% every night, but it seems to be slowing down ever so slightly by the day. (Sometimes I can feel the throttle trying to speed up but it won't).

Anyone else have this issue? Or any general tips for maintenance to keep it functioning at the speed I want? I store it inside, charge it fully, don't ride with extra people on it or anything, so not sure what could be done.